Long Term Evolution (LTE) of the 3rd Generation Partner Ship Project (3GPP) is the key technology of the next-generation cellular mobile communication. The physical layer uplink/downlink transmission scheme adopts a Single Carrier Frequency Division Multiple Access (SC-FDMA) with low peak-to-average ratio and an advanced and mature Orthogonal Frequency Division Multiple Access (OFDMA) technology respectively, based on which, Fast Link Adaptation and Multi-Input Multi-Output (MIMO) technology is introduced to improve the performance of LTE system.
The Physical Downlink Control Channel (PDCCH) bears the uplink and downlink control information of the whole system, and is closely related to the scheduling and allocation of resources of the whole system. Receiving of the channel plays a very important role in LTE system. Receiving of the PDCCH decides the delay of the whole system, thereby affecting the overall rate of reaction.
PDCCH bears the Downlink Control Information (DCI), including resource allocation and other control information of one or more User Equipments (UEs). Usually, there may be multiple PDCCHs in one subframe. UE needs to demodulate DCI in PDCCH first, and then demodulate the Physical Downlink Shared Channel (PDSCH) (including broadcasting message, paging, UE data, etc.) of UE at the corresponding resource location. A great variety of information can be transmitted over PDCCH during system operation, but which information can be transferred in each transmission is determined by the system deployment program. In LTE, the number of symbols occupied by PDCCH in one subframe is determined by the Control Format Indicator (CFI) defined in the Physical Control Format Indicator Channel (PCFICH). UE can determine the physical cell ID through primary/secondary synchronization signals, and determine the resource pattern of Physical Hybrid-ARQ Indicator Channel (PHICH) and system antenna port(s) and other contents through reading the Physical Broadcast Channel (PBCH). UE can then further read PCFICH and obtain the number of OFDM symbols occupied by the control channel like PDCCH. Besides PDCCH, the symbols occupied by PDCCH also include PCFICH, PHICH, and Reference Signal (RS), and so on, where PCFICH has been demodulated, allocation of PHICH is determined by PBCH, and pattern of RS is determined by the number of broadcasting antenna ports in PBCH. Thus, the Resource Elements (REs) occupied by all PDCCHs in one subframe can be determined.
Because a plurality of PDCCHs can be contained in the transmission bandwidth of PDCCH, in order to effectively configure the time-frequency resources of PDCCH and other downlink control channels, LTE defines two dedicated control channel resource units: RE Group (REG) and Control Channel Element (CCE). One REG is composed of 4 or 6 adjacent REs on the same OFDM symbol, but only 4 REs are available, the REG consisting of 6 REs includes two Reference Signals (RS), but the RE occupied by RS can't be available for the REG of the control channel. The protocol (36.211) also specifies in particular that when only one cell-specific reference signal is available, from the perspective of RE mapping in REG two antenna ports have to be assumed, so one REG may contain 4 REs or 6 REs. One CCE consists of 9 REGs.
PDCCH is transmitted in one or more of continuous CCEs; LTE supports 4 different types of PDCCHs, as shown in table 1.
TABLE 1PDCCH bitPDCCH formatCCE numberREG numbernumber01972121814424362883872576
In LTE, numbering and allocation of CCE is continuous. If the number of remaining REGs is NREG after the system allocates PCFICH and PHICH, the number of CCEs available by PDCCH is NCCE=NREG/9 rounded down. CCE is numbered from 0 to NCCE-1. The number of CCE occupied by PDCCH is determined by the downlink channel environment of UE. For the UE in a good downlink channel environment, the evolved NodeB (eNodeB) may only need to allocate one CCE; for the UE in a poor downlink channel environment, eNodeB may need to allocate as many as 8 CCEs. In order to simplify the complexity when UE decodes PDCCH, LTE also specifies that for the PDCCH which occupies N CCEs, the index of CCE at the start position of PDCCH must be an integral multiple of N.
When receiving the information sent by eNodeB. UE needs to monitor all PDCCHs in each subframe to detect whether the PDCCH contains the scheduling information or control information required, and needs to know the setting position of the CCE(s) corresponding to each PDCCH during monitoring. In order to describe such position information, that is, UE needs to monitor the position information of CCE candidate set, LTE defines the concept of search space, and classifies the search space into a Common Search Space and a UE-Specific Search Space. PDCCH candidates monitored by UE are shown in table 2.
TABLE 2AggregationSize ofNumber ofType oflevel ofsearch spacePDCCHsearch spacesearch space(with CCE as a unit)candidatesCommon search166space21264828162UE-specific4164search space8162
Each PDCCH contains 16 bits of Cyclic Redundancy Check (CRC) which is configured to validate by the UE whether the PDCCHs received are correct; CRC uses UE-related identity for scrambling, so that UE can confirm which PDCCHs are to be received and which PDCCHs are to be sent to other UEs. The UE Identity useable for scrambling is Random Access Radio Network Temporary (RNTI). After being checked by CRC, Tail Biting convolutional encoding and rate matching are performed to each PDCCH. ENodeB can perform rate matching according to the Channel Quality Indicator (CQI) reported by UE. Then, the number of CCEs occupied by each PDCCH can be confirmed.
The available CCE is numbered from 0 to NCCE-1. CCEs can be considered as logical resources and arranged sequentially to be shared by all PDCCHs. The eNodeB can place each PDCCH at an appropriate position according to the start position of CCE on each PDCCH. Under such a circumstance, some CCEs may not be occupied. According to the standard, NIL needs to be inserted (meaning void value), and the transmit power on RE corresponding to NIL is 0.
Then, data bits on CCE are subjected to physical cell ID-related scrambling, Quaternary Phase Shift Keying (QPSK) modulation, layer mapping and precoding, and the symbols obtained take Symbol Quadruplet (each Symbol Quadruplet is mapped to one REG) as a unit for interleaving and cyclic shift, and are finally mapped to the corresponding physical resource REG
Physical resource REGs are first allocated to PCFICH and PHICH and remaining ones are allocated to PDCCH for REG mapping following the principle of time domain first and frequency domain later. In this way, disequilibrium between PDCCH symbols can be avoided.
The current PDCCH blind detection method is designed for UE. In this method, the start position of DCI information is calculated through the given RNTI, then all aggregation levels are traversed once, and RNTI is used for CRC validation of the decoding result to acquire the DCI information of UE. Under the circumstance that RNTI of each user is unknown, if the current method is used, all RNTIs (ranging from 1 to 65535) need to be calculated. For the existing hardware level, the operation time is very long.